In a known system of this type for car radios, a microphone generates a control voltage which is applied to the volume control circuit of the radio. The sound output of the speaker is thus controlled by the microphone output voltage. A further voltage derived from the speaker input is applied to the volume control circuit in a direction opposite to the microphone voltage so that the gain of the volume control circuit will be inversely proportional to the voltage at the speaker input. To state it differently, the louder the speaker sound output, the less the increase in gain of the volume control circuit. Therefore, in the known apparatus, the gain of the volume control circuit depends upon the difference between the microphone output voltage and the speaker input voltage.
This type of apparatus has been found unsatisfactory for mobile radio receivers, and, in particular, for car radios. The ambient noise in an automobile changes very rapidly and over a wide range. For example, differences in vehicle speed, differences in traffic conditions (heavily trafficked streets or quiet side streets), etc. This calls for a large control range in the system. This in turn often results in an overdriving of the output stage normally connected to the output of the volume control circuit. A distortion of the sound output of the speaker results. This is often more disturbing to the listener than a decrease of the sound output of the speaker in relation to the noise level. Further, sudden increases in the speaker output can occur following pauses in the modulation, that is intervals where the speaker input signal decreases substantially or is totally eliminated. These increases in speaker output result from the fact that the speaker input voltage which acts in phase opposition to the microphone voltage was temporarily absent and that therefore a great increase in the gain of the volume control circuit was set. The speaker output immediately following such a pause in modulation is therefore very high and is returned to a comfortable volume only after reappearance of the speaker input voltage and thus of the opposing voltage for the gain control. Such an increase in the speaker sound output is very disagreeable, particularly when there are many such modulation pauses.